I own an epifone bass that I bought when I was child, and I'm trying to play again. It's active bass (or i think so since it has a 9 volt battery inside). The bass has two pickups and four knobs and I would like to know what are them for. One is the volume for shure Another seems to be some kind of another volume, don't know if it activates the active or something. The other two seem to be tones, but may be that tones one for each pickup? Both seem to do the same, bring the sound from very big round bass to very metallic lowcut bass. Any idea about that? Thanks a lot.